Transportation of substrate carriers within a fabrication facility, such as between substrate processing stations and/or from a storage location to a substrate processing station, may be accomplished at least in part via hoists, e.g., overhead transport (OHT) systems. OHT's may be designed to transport many substrate carriers simultaneously along a transport path that may include straight portions and/or curved portions. For example, straight portions of the transport path may run along a cleanroom aisle populated by successive mouths of cleanroom bays, or along a cleanroom bay containing successive factory interfaces of substrate processing stations. Accordingly, curved portions of the transport path may take the form of a turn out of a cleanroom aisle and into a cleanroom bay, or vice versa.
Such an OHT system may include a rotating element and carrier supports. For example, the rotating element may be suspended at an elevated height (e.g., in the overhead of the fabrication facility) and may be rotated through/along the transport path at that elevated height. Carrier supports may be coupled to the rotating element for receiving and supporting substrate carriers, and may be spaced at regular intervals, e.g., as may be determined based on the dimensions of the substrate carriers the OHT is to carry, taking into account such inter-carrier spacing as may be necessary to avoid mechanical interference during transport, load, and/or unload operations.
The functions of removing substrate carriers from the carrier supports of such an OHT (i.e., “picking”), and loading substrate carriers on carrier supports of such an OHT (i.e., “placing”), may require at least some minimum level of precision. For example, precision and/or predictability in the location, at any given time, of carrier supports of the OHT, or of substrate carriers supported thereby, may be needed. Also, good control over the movement of load and unload equipment relative to the OHT may be required. The accomplishment of important goals such as preventing uncontrolled shifting of substrates stored in the substrate carriers, and/or protecting the substrates from damage due to excessive shocks and/or impacts, may depend on the existence of such precision and/or such control. As such, methods and apparatus for providing improved control over the position and orientation of substrate carriers being transported along an inter-station transport path are desirable (e.g., control over the position and orientation of the substrate carrier relative to the rotating element of an OHT).